1. Field of the Invention
The present invention relates to the field of mandrels used in the production of hollow tubes, and more specifically, to a partially collapsible foam mandrel and method for making same.
2. Prior Art
The use of tubular fiber-reinforced resin composites is well known in the art. Such tubular composites have found special utility in the area of components used in the manufacture of helicopter blades. Such blades require extremely long, hollow tubes of very accurate dimensions and which are strong, yet substantially rigid. Because of the length involved and the high degree of accuracy required, the prior art methods of manufacturing such tubes have suffered from the disadvantage of being either expensive or not being capable of producing accurate tubes of this length. One prior art method for manufacturing short sections of tubular resin composites is to apply various piles or wrappings of resin impregnated fibers to a rigid cylindrical aluminum mandrel. The resin is allowed to cure and the aluminum mandrel must now be removed. If the tubular composite is relatively short, there is generally no difficulty in sliding the composite structure off the mandrel. However, one can easily see that when the composite to be formed has a long length, for example, in excess of 20 feet, it would be no simple task to withdraw the aluminum mandrel as the friction between the mandrel and the resin composite would be extremely great. Moreover, the use of the aluminum mandrel requires the fibers to be "wet wrapped", that is, the fibers are just wrapped on the mandrel and then a resin applied to the fibers. This process is messy and there is no way to achieve a uniform amount of resin on all the fibers.
Various concepts have been proposed to manufacture relatively long tubular composites. For example, one could attempt to apply resin impregnated fibers to an inflatable, semi-rigid tube such as a hose made of a butyrate material and the like. After the resin has cured, the semi-rigid tube could be deflated and removed leaving the tubular composite. The problem with this method however, is that the flexibility of the tube does not permit the requisite accuracy as the wrappings of fibers, when laid upon the semi-rigid tube, would have a tendency to compress the tube in various degrees and in various locations as the fibers are wrapped under tension. Moreover, a flexible tube 20 feet long would not have the necessary uniformity in diameter necessary for the manufacture of fiber tubular composites for the air craft industry.
Thus, the prior art methods, while they may be adequate for preparation of short sections of tubular composites, are completely useless when applied to the manufacture of relatively long members. The present invention represents an advancement in the art of mandrels and the manufacture of tubular composites, and contains none of the aforementioned shortcomings associated with the prior art production methods. The present invention provides a partially collapsible mandrel which permits the resin impregnated fibers to be applied thereto and then helically removed therefrom leaving a hollow tube of fibers and impregnated resin.